The invention relates particularly to electric heating elements comprising a tubular metal sheath, a resistance conductor within the sheath and connected to one end of a metal terminal pin, the other end of which extends outwardly of the open end of the sheath, compacted refractory material electrically insulating the resistance conductor and terminal pin from the sheath while being adapted to conduct heat from the resistance conductor to the sheath. Heating elements of this type are well known in the art and an example is found in U.S. Pat. No. 3 134 889, issued to L. D. Drugmand. End seals of various types have heretofore been provided in prior art heating elements, and the Drugmand patent discloses use of a plastic bushing.
In a prior U.S. Pat. No. (3 859 721) issued to me and Ralph Santora, a rubber bushing is used as the end seal and, although elastomeric bushings of this type proved satisfactory, their cost and the cost of assembling them as end seals was relatively expensive. In the highly competitive manufacture of electric heating elements, by high production methods, a savings of very little amount in cost often represents sales success, and efforts have been made to retain the benefits of an elastomeric end seal bushing but at reduced costs.
I have found, and commercial tests have proven, that a plastic bushing backed up by a fibre washer provides a high quality end seal for a sheathed electric heater at a cost below that of an elastomeric bushing and just slightly higher than the cost of a plastic bushing used alone. The heating element is adapted for water-heating purposes and the fibre washer is a protective element. Sometimes the water heating element is overloaded, or fails for some other reason, and this results in a buildup of heat which may melt the plastic bushing and may also cause fracture in that part of the sheath within the water. In such case, water under pressure will enter the sheath, but the fibre washer will absorb water and swell to form an even tighter seal to thereby restrict flow of water outwardly of the terminal end of the sheath.
Of the prior art presently known to me, only U.S. Pat. No. 2 703 834, issued to A. P. Charonneau, utilizes a fibre washer in the end portion of a tubular electric heater. In this patent, a pair of spaced fibre washers are used, but not for sealing purposes.